The present invention relates to an electroviscous fluid which is a fluid whose viscosity can be changed by varying an externally applied electric voltage. More particularly, the present invention relates to a thermally stable electroviscous fluid whose yield value is substantially increased by small voltages, which strongly resists particle sedimentation, and which does not wear or abrade surrounding equipment.
Fluids whose viscosity can be varied by the application of an external voltage can be used, for example, for power transfer, for shock absorption, and as valves, and as result such fluids have recently been the subject of some scrutiny. Fluids whose viscosity increases in response to an electrical field are collectively known as electroviscous fluids. However, in order to be able to withstand highly practical applications in, for example, clutches, engine mounts, and shock absorbers, an electroviscous fluid is required whose yield value is substantially increased by the application of small voltages.
Various types of such fluids have already been proposed. These have generally taken the form, for example, of dispersions of porous inorganic particles (e.g., silica, alumina, talc, etc.) in electrical insulating fluids. Electroviscous fluids are produced by dispersing in an electrical insulating fluid a particle having an electric double layer due to water being absorbed on the particle surface, the particles then orient in response to an external electrical field and the viscosity increases (more specifically, the fluid converts into a Bingham fluid and exhibits a yield value). This effect is hereinafter referred to as the Winslow effect. Based on the substantial practical advantages offered by silica (ease of industrial acquisition, rich potential for improvements and modifications in quality, etc.), many electroviscous fluids have been proposed that use silica in the system such as the fluids disclosed in U.S. Pat. No. 3,047,507 to Winslow and in Japanese Patent Application Laid Open [Kokai or Unexamined] No. 61-44998 [44,998/86]). However, these particular electroviscous fluids do not perform satisfactorily in industrial applications because they abrade the surrounding equipment, suffer from particle sedimentation, and exhibit a Winslow effect of modest degree.
In order to improve upon these deficiencies, electroviscous fluids have been proposed that comprise a dispersion of a polyelectrolyte in an electrical insulating fluid. The term polyelectrolyte collectively denotes polymeric compounds that contain ion pairs within the structure. Many natural and synthetic polyelectrolytes are known, and the ion-exchange resins are the best known. For example, Japanese Patent Application Laid Open No. 1-180238 [180,238/89] discloses an electroviscous fluid that comprises a dispersion in an electrical insulating fluid of 44,998/86 microparticles of a polyelectrolyte that contains amine salt structures. Japanese Patent Application Laid Open No. 1-262942 [262,942/89] discloses an electroviscous fluid that comprises a dispersion in an electrical insulating fluid of particles prepared by pulverizing an ion-exchange resin. There are several advantages associated with the use of such polyelectrolyte particles such as that since the particles are made of organic polymer, (a) they have a small specific gravity and thus resist sedimentation, (b) they have little abrasiveness for surrounding equipment, and (c) they provide a relatively large Winslow effect. Another advantage associated with synthetic polyelectrolytes is that the particle can be freely engineered.
However, the preparation of polyelectrolyte particle-based electroviscous fluids as known in the art involves solidification of the electrolyte through three-dimensional crosslinking by some methodology followed by the preparation of microparticles by, for example, pulverization. In this approach, the three-dimensional configuration of the electrolyte is locked in at the point of synthesis and the electrolyte can then no longer be reworked. Another deficiency in this approach is that the microparticulation process cannot produce perfectly spherical particles, and this in turn precludes both a satisfactory dispersion stability and a satisfactory Winslow effect. In addition, all of the polyelectrolyte particles proposed to date are carbon-based. Silicone oil as described below is the best electrical insulating fluid (dispersion medium), however, carbon-based particles have a poor affinity for silicone oil.